(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a digital color copier or the like, having image forming stations for forming images with multiple color toners in accordance with the color separated image information, a transfer means for superimposing and transferring the color separated images formed through the multiple image forming stations, a pattern image forming means for forming a predetermined pattern image by means of each image forming station, and a pattern image density measuring means for measuring the density of the pattern image, and in particular, relates to an image forming apparatus which can eliminate misalignment when superimposing individual color separated images so as to reproduce a correct color image.
(2) Description of the Prior Art
In conventional image forming apparatus, for example, in a digital color copier, color separated images of an original captured by the scanner are subjected to predetermined image processes so that the printer unit (image forming unit) produces a color image.
In this digital color copier, each of the color-separated images of the color original is recorded and reproduced separately so as to be superimposed onto a recording medium to thereby reproduce a full color image.
If the color separated images are not in proper register with each other when they are laid over one another, color misregistration occurs in the resultant image, hindering correct reproduction of the characteristics (image quality) of the original color image.
In order to make color separated images register with each other with a high precision on the recording medium, there recently have been digital color copiers which will execute a color misregistration correction periodically when the copier is in a predetermined state, to thereby produce an image representation which is closer to the original image.
Such color misregistration correcting schemes have been disclosed in Japanese Patent Publication Hei 7 No.19084 and Japanese Patent Application Laid-Open Hei 6 No.238954.
First, in Japanese Patent Publication Hei 7 No.19084, while color-separated measurement pattern images Bk, Y, M and Care formed on the conveyer and transfer belt, the passage of these color-separated measurement pattern images Bk, Y, M and C is detected and measured by a reflection type sensor. In this operation, the deviations of measurement patterns of color-separated measurement pattern images Bk, Y, M and C, from their set values are computed so as to be used to control the image forming (image writing) timing at which each image forming station forms its color separated image.
For computation of the deviations from their set values, measurement pattern image Bk formed by the black image forming station, being among color-separated measurement pattern images Bk, Y, M and C, is measured first by the reflection type sensor, then the number of timing signals are counted from the timing of the Bk detection up to the detection of other measurement pattern images Y, M and C, to thereby achieve measurement as to the deviation of image forming (image writing) positions by the different color image portions.
In the method disclosed in Japanese Patent Application Laid-Open Hei 6 No.238954, for a positioning pattern of an arbitrary color, multiple positioning patterns of the remaining colors which are created by successively translating the former pattern by one dot or by multiple dots, laterally and longitudinally (in the main scan and auxiliary scan directions) are stored beforehand, and the image having these patterns is output from the image forming stations.
Then, the timing at which one of the patterns (line pattern) of a remaining color matches the pattern (line pattern) of the arbitrary color is identified from the pattern image thus output so as to determine a value combination, based on which adjustment to the image forming (image writing) timing of each color image forming station is performed.
The method of Japanese Patent Publication Hei 7 No.19084, in which the positional (color) deviations of individual color-separated image formations (image writings) are measured by counting the number of timing signals, from when the reflection type sensor detects the reference measurement pattern Bk until the sensor detects each of the other measurement patterns Y, M and C, has the disadvantage in that correct color misregistration correction cannot be done due to the influence of driving irregularities in various driving portions (image forming stations and image transfer portions).
Since the method of Japanese Patent Application Laid-Open Hei 6 No.238954 is configured so that the image formed pattern image is viewed by the operator's visual observation and the result is input by the operator, only those skilled can make the adjustment, with still the possibility of input mistake. Further, this method have the problem of the paper being consumed to determine adjustment timing for every adjustment.